Because Saturn’s axis is tipped like the Earth’s, as it revolves around the sun in just over 29 years, we see the rings open and close. When the rings are edge-on to our view, Saturn is at or near one of its equinoxes. This happens approximately every 15 years. Illustration by Hui Chieh and used with permission.
Saturn revolves around the sun once every 29 years, and because it’s axis is tipped almost 27 degrees (just a little more than Earth’s), it experiences seasons. First one hemisphere and then the other is angled toward and then away from the sun, creating the familiar rhythm of high sun (summer) and low sun (winter). Since our planet takes just one year to complete an orbit, a season lasts a quarter of that or three months. Saturn takes 29 times as long to make the same journey, so each of its seasons lasts over seven years.
On Earth, the sun hovers directly over the equator only two times a year at the spring and fall equinoxes. The same is true at Saturn but because of its longer year, this only happens twice every 15 Earth years.
Saturn’s rings are some 175,000 miles across but only some 30 feet thick. In early August the sunlight just grazes the ring plane, causing anything poking above the rings to cast a long shadow. Illustration credit: NASA/CXC/M. Weiss
After a nearly 15 year wait, equinox time is fast approaching for the ringed planet as well. On August 11, the sun will shine straight over Saturn’s equator. Since the rings circle the planet in the same plane as the equator, sunlight will skitter across them at a very shallow angle. Any structures or tiny moons poking the smallest amount above the plane will catch the low sun and cast long shadows. Ever notice how even small things like gravel and grass cast considerable shadows near sunrise or sunset? The same thing is happening almost a billion miles away at Saturn. Low sun angles bring out details and features that would we’d otherwise never see.
So what might Saturn’s rings be hiding? The Cassini spacecraft has been watching the planet intently this summer making the most of the low sun. Below are several remarkable photos of teeny, tiny moons and their shadows as well as one unknown "structure" found in Saturn’s F ring.
The Cassini spacecraft captured this image of a small object in the outer portion of Saturn’s B ring casting a shadow on the rings as Saturn approaches its August 2009 equinox. The shadow length implies the moonlet is protruding about 660 feet above the ring plane. If the moonlet is orbiting in the same plane as the ring material surrounding it, which is likely, it must be about 1,300 feet across. All photos credit: NASA / Cassini Mission
Jagged looking shadows stretch away from vertical structures of ring material created by the gravitational pull of the moon Daphnis. Daphnis, 5 miles across, is a bright dot casting a thin shadow to the left and above the center of the image. How cool is this photo!
A vertically extended structure or object in Saturn’s F ring (bright, narrow band at far left) casts a shadow long enough to reach the A ring (right) in this Cassini image taken just days before planet’s August 2009 equinox. The structure can be seen as a bulge within the bright core of the F ring on the left of the image.
And now, just for your enjoyment, take a look at this stunning (and large) map of every object in the solar sytem larger than 200 miles across. There are many of them!